1. Technical Field
The present invention relates generally to gallium nitride substrates, and to methods of testing and manufacturing gallium nitride substrates.
2. Description of the Related Art
Gallium nitride substrates are ordinarily manufactured in the following way, as taught, for example, in Japanese Pat. No. 3581145. Initially, an ingot composed of gallium nitride is subjected to a grinding process to make it cylindrical. Next, a notch or orientation flat is formed on the lateral side of the cylindrical ingot for determining its crystal orientation. The notch consists of a V-shaped indent, while the orientation flat is a surface ground parallel to a predetermined crystal plane. Subsequently, the ingot is sliced using a cutting device such as an inner-circumferentially bladed slicer, or a wire saw, to yield a gallium nitride substrate. After that, the substrate edges are chamfered, and then a lapping (mechanical polishing) process is carried out on the substrate back side. The manufacturing procedure continues with a polishing operation (planarization process) carried out on the substrate front side to produce a mirrorlike finish, after which the substrate is washed. A gallium nitride substrate provided with a mirrorlike surface on which devices can be formed is thereby produced.
When the back side of a gallium nitride substrate is finish-processed, it is common for contaminants to cling to the back side, and for a damaged layer containing distortions and cracks to form on the back side. Gallium nitride substrates are consequently susceptible to fracturing, such that improving manufacturing yields has been difficult.